drivers/espi: it8xxx2: Fix mask when SRAM size not the power of 2

When using the it8xxx2bx chip, because the SRAM size is 60k and
not a power of 2, using (CONFIG_SRAM_SIZE - 1) results in an
incorrect value for the h2ram base address mask.
Therefore, the mask is declared as GEMASK(19, 0) here.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>