spl: nand: Set bl_len to page size

spl: nand: Set bl_len to page size

Since commit 34793598c83 ("mtd: nand: mxs_nand_spl: Remove the page aligned
access") there are no longer any users of nand_get_mtd. However, it is
still important to know what the page size is so we can allocate a
large-enough buffer. If the image size is not page-aligned, we will go off
the end of the buffer and clobber some memory.

Introduce a new function nand_page_size which returns the page size. For
most drivers it is easy to determine the page size. However, a few need to
be modified since they only keep the page size around temporarily.

It's possible that this patch could cause a regression on some platforms if
the offset is non-aligned and there is invalid address space immediately
before the load address. spl_load_legacy_img does not (except when
compressing) respect bl_len, so only boards with SPL_LOAD_FIT (8 boards) or
SPL_LOAD_IMX_CONTAINER (none in tree) would be affected.

defconfig               CONFIG_TEXT_BASE
======================= ================
am335x_evm              0x80800000
am43xx_evm              0x80800000
am43xx_evm_rtconly      0x80800000
am43xx_evm_usbhost_boot 0x80800000
am43xx_hs_evm           0x80800000
dra7xx_evm              0x80800000
gwventana_nand          0x17800000
imx8mn_bsh_smm_s2       0x40200000

All the sitara boards have DDR mapped at 0x80000000. gwventana is an i.MX6Q
which has DDR at 0x10000000. I don't have the IMX8MNRM handy, but on the
i.MX8M DDR starts at 0x40000000. Therefore all of these boards can handle a
little underflow.

Signed-off-by: Sean Anderson <seanga2@gmail.com>